1. Field of the Invention
The invention relates to a process and in particular to a process for preparing bisphenol A.
2. Description of Related Art
2,2-Bis(4-hydroxyphenyl)propane (bisphenol A, BPA), which is the condensation product of phenol and acetone, is a starting material or intermediate for the preparation of a large number of commercial products. Thus it is used as a starting material for the preparation of various types of polymeric materials, for example polyarylates, polyetherimides, polysulfones and modified phenol-formaldehyde resins. Preferred fields of application are in the preparation of epoxy resins and polycarbonates.
Technically relevant methods of preparing BPA are known, e.g. WO 00/35847 and U.S. Pat. No. 2,775,620.
After the acid-catalyzed reaction of phenol with acetone, BPA is separated from the product mixture in the form of a crystalline adduct of BPA and phenol. Phenol is completely or partially removed from these crystals of BPA-phenol adduct by distillative, desorptive or extractive methods. Before the separation from phenol, the crystals of BPA-phenol adduct may also be subjected to additional purification steps to reduce the concentration of secondary components.
The crystals of BPA-phenol adduct may be recrystallized e.g. from phenol, organic solvents or mixtures of these solvents. The phenol present in the adduct crystals may also be completely or partially separated off using an appropriate solvent.
PL 159620 describes a laborious process for the purification of bisphenol A wherein the crystals of BPA-phenol adduct isolated in a first stage are dissolved in phenol and the resulting solution is cooled in a batch crystallizer in a system similar to a static multilayer crystallization process, wherein crystals of BPA-phenol adduct growing on the tubes. When the mother liquor has been drained off, the crystals of BPA-phenol adduct recrystallized in this way are washed with phenol and water in order to remove any mother liquor still adhering to crystals. Alternatively, or in addition, the crystals can be purified further by heating to a temperature of 75 to 85° C., i.e. by exudation. However, as the process has to be carried out batchwise, equipment costs are typically high. Moreover, the production outputs based on the heat-transfer area are an order of magnitude lower in a process according to PL '620 than in a continuous suspension crystallization process used to produce crystals of BPA-phenol adduct. Furthermore, an acceptable space-time yield requires high cooling rates, so a multilayer crystallization of this type has to take place at substantially higher crystal growth rates than a continuous suspension crystallization. The resultant increased incorporation of impurities into the crystals has to be compensated by time-intensive exudation and/or washing operations. If a high purity is required, it may be necessary to use a multistage process.
EP 718 268 A describes a process for the preparation of a crystalline adduct of bisphenol A and phenol wherein BPA dissolved in phenol is first crystallized out as an adduct and filtered off. It is then redissolved in phenol, crystallized out again and filtered off, and the step of recrystallization being repeated several times. In the last step of the multistage recrystallization, the adduct is washed with specially purified phenol. The washing liquids and filtrates for washing and dissolving the adducts are at least partially recycled.
WO 02/40435 describes a process wherein high-purity bisphenol A is obtained by multiple recrystallization from phenol and washing of the adducts in individual stages with high-purity phenol in a cross-flow washing plant. The phenol used as washing phenol contains virtually no bisphenol A or isomers thereof. A multiple recrystallization is preferred. Washing with high-purity phenol at each of the individual stages requires a very large amount of high-purity phenol. Furthermore, a large amount of bisphenol A is dissolved in the phenol at each individual stage, resulting in losses of yield.
WO 03/82785 describes a process for the preparation of bisphenol A wherein in the first step a suspension of crystals of BPA-phenol adduct is filtered under vacuum and washed, and the resulting filter cake is dissolved in a liquid containing phenol and crystallized again. The crystals obtained are separated off by centrifugation. However, the continuous (filter) centrifuges that are more economical for larger plant capacities demand a minimum crystal size, which requires special crystallization procedures.
EP 1 367 043 A describes a process for the purification of bisphenol A wherein the crystals of BPA-phenol adduct are dissolved in phenol and the resulting solution is filtered at least once before the crystals of BPA-phenol adduct are crystallized out again.